Electron gun device having a field emission cathode tip protected from destruction due to ion impingement

ABSTRACT

In an electron gun device of the field emission type, a suppression electrode having a potential which is higher than that of the anode is disposed between a cathode tip of needle shape and said anode so that passage through said suppression electrode of ions emitted from said anode by means of electron impingement thereon is prevented, whereby the needle tip of the cathode is protected from destruction due to ion impingement thereon.

United States Patent 1191 1111 3,846,663

Komoda 1 Nov. 5, 1974 [54] ELECTRON GUN DEVICE HAVING A FIELD 2,363,35911/1944 Ramo 313/82 R X EMISSION CATHODE TIP PROTECTED 3,384,032 1945(Bioodrich 313/82 R 5:

,888,6 1959 earn 313 82 R FROM DESTRUCTION DUE To ION 3.141993 7/1964Hahn 313/82 R x IMPINGEMENT 3,191,028 6/1965 Crewe 250/311 x [75]Inventor; Tsutomu Komoda, Kodaira, Japan 3.614520 10/1971 Coleman 313/82R X 3,678,333 7/1972 Coates et a1. 250/311 X [73] Assignee: Hitachi,Ltd., Tokyo, Japan Prima ExaminerRobert Se al 22 F1 g 1 1 ed Sept 1971Attorney, Agent, or FirmCraig & Antonelli [21] Appl. No.: 182,041

[] Foreign Application Priority Data ABSTRACT Sept. 18, 1970 Japan..45-8l283 I an electron gun device 0f the field emission type, asuppression electrode having a potential which is [52] US. Cl 315/3,2530l/g/l8l6, 33ll3sl/832liiz, higher than that of the anode is disposedbetween a [51] In C] 29/02 29/52 cathode tip of needle shape and saidanode so that [58] Fie'ld 5 R ZgO/BIO passage through said suppressionelectrode of ions 5 5 315/3 emitted from said anode by means of electronimpingement thereon is prevented, whereby the needle tip of the cathodeis protected from destruction due to [56] UNITE A E S :ZqrENTS ionimpingement thereon. 2,269,613 1/1942 Baeyer et a1 313/82 x 2 3 D'awmgF'gms H 2/1942 Ramberg et al. 313/146 X PATENTEDNUV sum 10: 2

FIG. I

PRIOR ART FIRST HIGH VOL- TAGE SOURCE I SECOND HIGH VOL- TAGE SOURCE VFIRST HIGH vo| TAGE SOURCE v 0.0 VOLTAGE SOURCE v2 SECOND HIGH VOL- TAGESOURCE V0 am: 5 m4 3; PAIENTEIJ sum 2 M 2 84B 663 ELECTRON GUN DEVICEHAVING A FIELD EMISSION CATHODE TIP PROTECTED FROM DESTRUCTION DUE TOION IMPINGEMENT BACKGROUND OF THE INVENTION This invention relates to anelectron gun device of the field emission type, which is particularlysuitable as a source of an electron beam in an electron microscope, andthe like.

As is well known, a conventional electron gun device of the fieldemission type for an electron microscope has a cathode tip of needleshape, means for heating said tip and an anode for causing electrons tobe emitted from said tip by creating an electric field between said tipand said anode.

In such an electron gun device, when it is intended to obtain anelectron beam having a current value which is more than I011. A, theelectron beam varies unstably with a frequency which increases rapidlywith operation. As a result of this, the tip portion of said cathode isdestroyed so that the field emission of electrons therefrom stops.

Since the useful life of the cathode tip in the conventional electrongun device is very short for the reasons described above, it isnecessary to exchange said tip frequently. In the electron gun device,ultra high vacuum conditions must be maintained and once this conditionis broken for exchange of said tip, it is necessary to evacuate theelectron gun device once again. However, this requires complicatedprocedures and a very long time is required to effect such evacuation.

SUMMARY OF THE INVENTION An object of the invention is to provide anelectron gun device having a very long useful life.

Another object of the invention is to provide an electron gun devicehaving the advantage that the number of times wherein exchange of thecathode tip is required, and thus the number of times wherein evacuationof the microscope is necessary, has been greatly reduced.

A further object of the invention is to provide an electron gun devicesuitable as the source of an electron beam for use in an electronmicroscope and the like.

In order to attain the above objects, the electron gun device of thisinvention is characterized by employment of a suppression electrodedisposed between the cathode tip and the anode which will prevent thepassage therethrough of ions emitted from said anode as a result ofimpingement of the electrons thereon.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram showinga conventional electron gun device; and

FIGS. 2 and 3 are schematic diagrams showing embodiments of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, an electron gun ofthe field emission type for an electron microscope includes a cathodetip 1 of needle shape for electron emission, a tungsten filament 2 ofhair-pin shape, terminal electrodes 3, a first anode 4, a second anode6, the respective anodes being provided with holes 5 and 7 through whichan electron beam passes. A first high voltage source 8 is connectedbetween the tip 1 and the first anode 4. A second high voltage source 9is connected between the tip 1 and the second anode 6. The connectionpoint of the second anode 6 and the positive terminal of the voltagesource 9 is grounded.

In the conventional electron gun structure, a negative high voltage, forexample -30KV, is applied to the cathode tip 1 through the terminalelectrodes 3, and another high voltage of 2-3KV is applied between thetip I and the first anode 4 by the source 8 connected therebetween. Inthe electron gun device of conventional structure, since the intensityof the electric field adjacent to the tip portion of the cathode tip Ihas a very high value due to the potential difference between said tip 1and the first anode 4, electrons are caused to emit from said tip 1 fromthe electric current flowing through the filament 2 so as to heat it andclean its surface. These electrons passing through the hole 5 arefocused by the electric field produced between the first and secondanodes 4 and 6 which act as an electron lens so that an electron beamhaving a fine spot of high density is obtained from the hole 7.

Now, in this case, since an electric field is formed as shown by dottedlines in FIG. 1, electrons l1 emitted from the tip I, as shown in thesame figure, are accelerated by the potential difference between thecathode and anode 4 and all electrons except those passing through thehole 5 impinge on the first anode 4.

Molecules on the surface of the first anode 4 which are struck by theseelectrons are ionized and these ions are emitted from the surface due tothe potential gradient in the interelectrode space and the force of theelectron impingement thereon. Emitted ions are accelerated by thepotential difference between the electrodes in the opposite direction ofthe electrons and impinge on the tip portion of the cathode tip 1. Anatomic layer is etched on the surface of said tip portion as a result ofthis bombardment and its surface condition is varied by means of the ionimpingement thereon. As a result of this, the condition of the fieldemission from the cathode tip 1 is varied so that emission of theelectron current thereon becomes unstable.

Generally, the tip portion of the cathode top 1 is very sharp so as toallow electrons to be emitted therefrom by means of electric fieldeffects. For example, a tungsten line, having a diameter of 0.l 0.2mmand a radius of curvature at the tip portion formed in needle shape tohave the very small value of 500 A., is used as the tip. Therefore, whenthe ion impingement on the tip exceeds a certain limit, the tip portionof the cathode tip is destroyed so that emission of electrons therefromstops.

Since the tip 1 is destroyed for reasons described above, it is apparentthat the tip can be protected from such destruction by preventing theion impingement thereon. Thus, the electron gun device of the presentinvention is designed to have a suppression electrode for preventing thepassage therethrough of ions; for example, an electrode 12 of mesh shapehaving large openings is disposed between the tip 1 and the first anode4, as shown in FIG. 2, and to which a positive high voltage having avoltage value of 10 V higher than that applied to the first anode 4 isapplied from a DC voltage source 13 whose a positive terminal isconnected to said electrode 12.

As one example, the voltages V0, V1 and V2 of the respective voltagesources 9, 8 and 13 are set at V 30,000V, Vl 2,500V and V2 lOOV. Withsuch an arrangement, since a positive high voltage having a voltagevalue of V1 V2 with respect to the tip is applied to electrode 12,electrons are emitted from the tip 1 in a manner similar to aconventional electron gun device. The emitted electrons reach theelectrode 12, but since this electrode has very large openings arrangedin the form of a mesh, they are not intercepted by it so that a majorpart of said electrons pass through said electrode 12 and reach thefirst anode 4.

All electrons except those passing through the hole 5 impinge on thesurface of the first anode 4 and thereby ions are produced therefrom;however, since the electrode 12 has a positive potential with respect tothe first anode 4, the potential gradient between the electrode 12 andthe first anode 4 will not accelerate the ions away from the anodesurface. In fact, the ions generated by the electron bombardment of thesurface of anode 4 will be repelled by the electrode 12, therebypreventing the ions from reaching the cathode tip. Thus, the tip portionof the cathode tip 1 will not be destroyed with the result that said tipwill have a very long and useful life.

in the above embodiment, the mesh electrode 12 is used as a suppressionelectrode, as described above, but another electrode 14 having a singlelarge opening can be used instead of said mesh electrode, as shown inFIG. 3. In FIG. 3, the voltage sources 8, 9 and 13 are omitted, but therelation between the voltages applied to the respective electrodes issimilar to that in the aforementioned embodiment.

It is sometimes difficult to produce a uniform potential distribution bymeans of the mesh electrode 12; however, since the electrode 14 can bemade precisely and has the single large opening 15, it is possible toproduce a uniform potential distribution along an axis 16, and thus theaberration is made small.

Electrons are emitted from the tip within the limits of an opening anglea of about 30 therefrom due to the shape of the tip portion. Therefore,if the opening 15 of the electrode 14 is made in such a manner as tohave such an opening angle, the emitted electrons will not impinge onthis electrode, and therefore will reach the first anode 4. in a mannersimilar to the embodiment shown in FIG. 2, ions produced at the firstanode 4 cannot pass through said electrode 14 on account of thepotential distribution as shown in dotted lines in FIG. 3.

As is apparent from the above description, according to the presentinvention, the top portion of the cathode tip can be protected fromdestruction due to ion impingement, and therefore the life time of saidtip greatly increases. Moreover, this invention can be applied to allelectron gun devices of the field emission type having at least acathode tip of needle shape and an anode for producing the electricfield between it and the tip, so as to cause electrons to be emittedfrom the tip.

What I claim is:

1. In an electron gun device of the field emission type having a cathodetip of needle shape, means for heating said cathode tip to clean it,means for drawing electrons from said cathode tip including an anodeopposing said cathode tip and having a hole therein which is of smallerarea than the area of the remainder of said anode through which at leastpart of the electrons from said cathode pass and means for applying apositive electric voltage to said anode with respect to said cathode tipso as to produce an electric field therebetween sufficient to causeelectrons to be emitted from said cathode tip toward said anode, theimprovement which comprises suppression electrode means disposed betweensaid cathode tip and said anode for preventing passage therethrough ofions emitted from said anode by means of electron impingement therein,said suppression electrode means including an electrode having anelongated annular configuration symmetrical about a central axis with amiddle portion thereof being closer to said central axis than the twoend portions thereof, and

means for applying to said suppression electrode means a positivevoltage higher than that applied to said anode, whereby said cathode tipcan be protected from destruction due to ion impingement thereon.

2. An electron gun device as defined in claim 1, wherein saidsuppression electrode has a single large opening having an opening angleof about 30 measured from said cathode tip toward said anode.

1. In an electron gun device of the field emission type having a cathodetip of needle shape, means for heating said cathode tip to clean it,means for drawing electrons from said cathode tip including an anodeopposing said cathode tip and having a hole therein which is of smallerarea than the area of the remainder of said anode through which at leastpart of the electrons from said cathode pass and means for applying apositive electric voltage to said anode with respect to said cathode tipso as to produce an electric field therebetween sufficient to causeelectrons to be emitted from said cathode tip toward said anode, theimprovement which comprises suppression electrode means disposed betweensaid cathode tip and said anode for preventing passage therethrough ofions emitted from said anode by means of electron impingement therein,said suppression electrode means including an electrode having anelongated annular configuration symmetrical about a central axis with amiddle portion thereof being closer to said central axis than the twoend portions thereof, and means for applying to said suppressionelectrode means a positive voltage higher than that applied to saidanode, whereby said cathode tip can be protected from destrUction due toion impingement thereon.
 2. An electron gun device as defined in claim1, wherein said suppression electrode has a single large opening havingan opening angle of about 30* measured from said cathode tip toward saidanode.